Field of the Invention
This invention relates to an oxygen sensor and more particularly to, an oxygen sensor element in which the resistance of the element varies depending on a concentration of oxygen in a multicomponent gas environment.
In order to control the air-to-fuel ratio in a combustion system of automotive internal engines or in burners or to operate safety devices for preventing the incomplete combustion of gas or oil machinery and apparatus, there has recently been an increased demand for oxygen sensors which are inexpensive and highly reliable in performance.
Typical of commercially available oxygen sensors is an oxygen sensor which comprises a sensor substrate or base of a specific form composed of a solid electrolyte of stabilized zirconia and a pair of platinum electrodes disposed along the inner and outer surfaces of the substrate, respectively. In the sensor of the just-mentioned type, the concentration of oxygen in a gas mixture is detected as a change in electromotive force generated between the electrodes when a gas composition is changed. This sensor has an advantage that even though oxidative and reductive gases are both present in a gas environment to be measured, the chemical equilibrium reaction proceeds rapidly owing to the catalytic action of platinum used as the electrodes, so that an abrupt change of the electromotive force of the sensor takes place in the neighbourhood of the stoichiometric point of combustion of both types of gases. However, it is disadvantageously expensive since the platinum electrode is essential and the sensor has to be constructed in a complicated form.
Another type of oxygen sensor has been also proposed using a substrate constituted of TiO.sub.2 or MgCo.sub.2 O.sub.4, in which a variation in resistance of the substrate is utilized for detecting an oxygen concentration in a gas mixture. Though inexpensive, this sensor itself has no function as a catalyst for combustion and when it is used in an atmosphere of an exhaust gas where oxidative and reductive gases are present, its performance becomes unstable in the vicinity of the stoichiometric point of combustion. Thus, an abrupt change of the resistance at that point cannot be expected. In the cases of automotives and large-size steam power plants provided with three-way catalyst systems for converting harmful gases in exhaust gases into innoxious ones, satisfactory results are obtained when the sensor is placed in a passage of an exhaust gas which has been passed through the three-way catalyst system to undergo the chemical equilibrium reaction. However, combustion devices of small sizes which have no catalyst system encounter the problem mentioned above.